This invention relates in general to optical devices and more particularly to a bond for attaching a crystalline or vitreous optical element to a heat sink that dissipates heat from the element and to method of making the bond.
A solid state laser rod, before it will emit electro-magnetic radiation, must be excited by an outside source of energy. This source of energy emits visible or near visible radiation itself which is converted by the laser rod into a laser beam. However, solid state laser rods exhibit relatively small gain, that is, they will convert only a relatively small portion of the energy directed onto them into laser radiation. Much of the remaining energy is converted into heat, and this presents a serious problem because most solid state laser rods operate effectively only below about 30.degree. C., and 50.degree. C. is about the maximum temperature for operation. Above 50.degree. C. they will not lase at all.
Thus, in order for a laser rod to operate at all some provision must be made to dissipate the heat which is generated. Solid state laser rods are presently cooled by water circulated around them, but water cooling presents problems of its own and furthermore is not practical in many applications. Laser rods are also cooled by conducting the heat away from them through their mounts.
Heretofore conductive cooling has presented a wide variety of problems and has not been entirely satisfactory. These problems reside primarily the in bond between the rod and its mount or heat sink as it is often called. Perhaps the most suitable procedure heretofore developed for producing the bond involves depositing a metal on the surface of the rod, depositing another metal on the surface of the heat sink, and soldering the two deposited metals together with a compatible solder. For example, in the case of a yttrium aluminum garnet (YAG) laser rod, gold is deposited on a selected area of the rod by vacuum deposition. Likewise a thin layer of gold is applied to the surface of the mount against which the rod is to be positioned. Usually the mount is copper. Then the mount and rod are joined together at their gold coatings with indium or some other solder.
The foregoing process provides neither a very reliable nor a very predictable bond. Through evaluations made in connection with the development of this invention, the problems in current bonds seem to derive from the tendency of the solder to scavenge some of the metal coating on the rod. Thus, the metal coating does not exist over the entire rod area that is located opposite to the heat sink and is not as strong as it might otherwise be. Also, these bonds tend to degrade with the passage of time. Furthermore, the scavenged areas are not nearly as reflective as the coated areas, and this lowers the gain of the laser rod, for light rays that might otherwise be reflected into the rod and converted into a laser beam are merely absorbed in the solder. Since the scavenged areas vary from mount to mount and even between successive attachments to the same mount, it is impossible to compare laser rods for gain and other characteristics. In other words, any tests run on laser rods mounted in such a manner reflect the gain or efficiency of the rod and its mount and not the rod itself.